I solved this issue by myself.
First I tried if I am able to access https://nextcloud.<mydomain>.<tld>/.well-known/caldav
from the affected iPhone, which was successful after entering the right credentials.
Then I removed the NextCloud configuration profile from the iPhone (see > Settings > General > VPN & Device Maintenance
) and get a new one by using the NextCloud-UI-Function by clicking on the profile button in the upper right corner, and then navigating to > Settings > Mobile & Desktop > Download macOS/iOS Configuration Profile
and installed it.
Note: The accounts password has to be entered twice during the profile installation process since not only the CalDAV- but also the CardDAV-connection will be installed.
After that renewal the Cal- and CardDAV-synchronisation works well again.
Unfortunately I did not store the old configuration profile file since in this case it would be possible to compare the old and the new one to get an idea about the problem with the old profile in combination with NextCloud version 28.0.1
I hope that this may help other people here in the community.